Retirement savers putting money to work today's investing environment should expect as little as a 4 percent return over the long term, said Larry Fink, chairman and CEO of BlackRock, the world's largest asset manager.

"It would be wrong to expect anything more than 4 percent or 5 percent at this time, if you're putting money to work today for long term," he told CNBC's "Squawk Box" on Monday, based on a typical retirement portfolio that includes stocks and bonds.

"If you go into that without that assumption, you'd be wrong," said Fink, adding that the stock market alone should still return the historical average of 6 to 7 percent over time. BlackRock has about $4.5 trillion in assets under management.

The U.S. stock market could see "the beginning of another leg of a rally" if Britain votes this week to stay in the European Union trading block, Fink predicted.

U.S. stocks soared at the open Monday, as global markets rallied on polls showing the stay camp making gains ahead of Thursday's Brexit vote. Last week's pessimism over the referendum knocked Wall Street lower.

BlackRock on Monday upgraded its short-term view on U.S. Treasurys and fixed-income overall to neutral, while remaining cautious on risk assets ahead of the Brexit vote.

"If the U.K. votes to stay, I do believe there has to be policy responses," Fink said. "I'm under the belief now that the government [there] is aware that this anger is real. And if they don't respond to this anger it's going to get worse."

Fink said that if Britain can get its fiscal house in order, that might inspire the U.S. to break the partisan gridlock in Congress, which would be a positive for U.S. stocks.

"The U.K. has to begin a major fiscal policy expansion, especially in infrastructure," Fink said, drawing a parallel between the anger and fear among the British electorate and "what's going on here [in the U.S.] in our election cycle."

Fink said he's "still nervous" ahead of the U.K. vote. "It's always going to be a close vote. And I hope ... [a Brexit] doesn't happen."

Either way the vote goes, the next step for the U.K. is that the government needs to "create opportunity" to ease concerns people have about their economic future, he said.

Concerns last week pushed the yield on the 10-year government bond in Germany into negative territory for the first time ever. The Bund yield has since turned positive.

Negative interest rate policies by central banks in the euro zone and in Japan are doing "more harm than help," Fink said. "The rates are going negative artificially [in the market] by the purchase of all the bonds by all these central banks."

But Fink added that central bankers have had little choice to use extraordinary measures, considering they've been "only game in town." He called them "bold citizens of the world."

Monetary policymakers do need to be more vocal in rallying help from governments who "abdicated in most countries any responsibilities for growth," he added.

Sign Up for Our Newsletter
Morning Squawk

CNBC's before the bell news roundup

SIGN UP NOW

Get this delivered to your inbox, and more info about about our products and service.Privacy Policy.